Surface structure and composition play a significant role in determining the physicochemical properties of solid materials. Of particular interest are fundamental material properties such as wetting, adsorption, adhesion, friction, electrostatic charging, and biocompatability. The ability to alter the chemistry and structure of surfaces is accordingly of great technological significance.
One approach to the chemical modification of solid surfaces is via the deposition of Langmuir-Blodgett films as disclosed in the following literature: K. B. Blodgett, J. Am. Chem. Soc., 56:495 (1934); K. B. Blodgett, J. Am. Chem. Soc., 57:1007 (1935); K. B. Blodgett and I. Langmuir, Phys. Rev., 51:964 (1937); G. L. Gaines, Jr., Thin Solid Films, 68:1 (1980); G. G. Roberts, Contemp. Phys, 25 (2):109 (1984); and U.S. Pat. No. 4,554,076, of Speaker.
Langmuir-Blodgett (hereinafter referred to alternatively as "L-B") films are ultra-thin organic films of molecular dimensions fabricated from the sequential transfer of monomolecular layers of surface-active molecules previously cast onto a liquid surface. The floating precursor films, (called Langmuir films or spread monolayers), comprise surface active molecules such as long chain fatty acids, lipids, phospholipids, proteins, polymers, porphyrins, and the like, with ionizable or non-ionizable polar head groups and lipophilic tail segments. Such materials, having a combination of both hydrophilic and hydrophobic properties are commonly referred to as amphiphilic materials.
In the formation of Langmuir-Blodgett thin films, the surface active material is first prepared as a dilute solution in a suitable organic carrier solvent such as heptane or chloroform. The solution is then applied to a surface of a Langmuir trough so as to spread out mono-molecularly thereupon. This monolayer film is subsequently compressed to yield a pseudo-solid film. In one method, compression is achieved by moving the subphase beneath the film as described in Japanese Patent 60-193532. Alternatively, compression can be achieved by moving the gaseous phase above the film or by moving a solid barrier at the film periphery.
A typical Langmuir trough comprises a means for containing the liquid subphase. A working area is defined by a barrier (for example, a PTFE-coated glass fiber) which defines the area to be occupied by the monolayer. This barrier can be moved using a means such as a low-geared electric motor. By moving the barrier to increase or decrease the area defined therein, it is possible to maintain a constant barrier layer pressure as the volume of the monolayer material is decreased via deposition onto the substrate.
The barrier motor can be coupled to a sensitive electronic balance which continuously monitors, via means such as a sensing plate, the surface pressure of the monolayer. Using feedback monitoring, the monolayer pressure can be maintained at a predetermined value.
The physical dimensions of the Langmuir trough are largely arbitrary, being governed only by the size of the substrate upon which the film is to be deposited. Strict cleanliness conditions are considered necessary to produce films of uniform quality. As such, Langmuir-Blodgett film deposition systems are usually maintained in controlled environments such as within glove boxes, laminar flow hoods, or in clean-rooms such as those used for pharmaceutical or microelectronic fabrication.
The solid substrate onto which the film is to be deposited is made to cut the surface of the monolayer-bearing subphase, usually vertically or at an angle, by lowering the solid substrate into the liquid subphase through the monolayer. In so doing, the compressed monolayer film is transferred to the solid substrate. By reciprocating the solid substrate through the surface of the subphase, several monolayers can accordingly be built-up onto the solid support. This technique is commonly referred to as the Langmuir-Blodgett Technique and the films formed thereby are commonly referred to as Langmuir-Blodgett films.
The deposition of a Langmuir-Blodgett (L-B) film occurs in such a manner as to result in the formation of one of three basic types of organized structure. These structures are commonly referred to as x-, y-, and z-type films, the most common being the y-type. In a y-type film, the immersion of a hydrophobic substrate into the monolayer-containing subphase results in a strong hydrophobic association between the hydrophobic surface of the substrate and hydrophobic terminals of amphiphilic monolayer entities. The hydrophobic attachment results in the exposure of the hydrophilic terminals of the monolayer entities on the immersed film. When the film is raised through the monolayer, however, the exposed hydrophilic terminals associate with hydrophilic terminals of the monolayer entities to result in a film having an exposed hydrophobic surface. As the film is gradually built-up by repeated traversal of the monolayer surface, the exposed film surface upon removel from the subphase tends to exhibit continued hydrophobic-properties.
In less common configurations, such as in the x-type structure, a weak hydrophobic interaction between the substrate and the hydrophobic terminals of monolayer entities first occurs. Subsequent monolayer traversals either into or out of the subphase liquid result in a weak hydrophobic-hydrophilic interaction between the hydrophilic head groups of one film layer with the hydrophobic tail groups of a subsequently deposited film layer. Similarly, in the z-type film structure, an initial film layer is deposited through a weak association between the hydrophobic surface and hydrophilic terminals of monolayer entities. This results in a surface having exposed hydrophobic terminals. Subsequent layers are deposited by similar hydrophobic-hydrophilic associations resulting in a built-up film having a hydrophobic surface characteristic. Thus, the z-type structure, like the y-type structure, provides a film having hydrophobic surface properties. In contrast, x-type films, which are deposited by interaction of hydrophobic terminals with the underlying film layer, have hydrophilic surface properties.
For many years, these films were generally regarded as little more than an academic curiosity (G. L. Gaines, Insoluable Monomers at Liquid Gas Interface, Interscience Publishers, N.Y. (1966)). In recent times, however, it has become apparent that these films can have great utility in areas as diverse as electronics (P. S. Vincent and G. G. Roberts, Thin Solid Films, 68:135 (1980)); tribology (U.S. Pat. No. 4,548,873 of Yamamoto et. al.); molecular electronic devices (G. G. Roberts, Sensors and Actuators, 4:131 (1983) and G. G. Roberts, Adv. Phys., 34 (4):475 (1985)); molecular signal generation and processing (H. Kuhn, Naturwiss, 54:429 (1967) and H. Kuhn, Photochem. 10: 111 (1979)); optical devices (B. Liedberg et. al., Sensors and Actuators, 4:299 (1983)); and electro-optics (D. B. Neal et. al., Electronics Letters, 22 (9):460 (1986) and G. M. Carter et. al., Optical Engineering, 24 (4); 609 (1985)).
In many technological applications of L-B films, there is a need to achieve a topologically uniform, ultra-thin organic overlayer of controlled and uniform surface chemistry. However, the fabrication techniques associated with L-B films often yielded hydrophobic surfaces and these hydrophobic surfaces were generally chargeless and unreactive. As such, these films to date have generally been limited to applications in which hydrophobic surface characteristics are desirable.